Recently, a metallic belt-type continuously variable transmission and a toroidal continuously variable transmission have been developed as a transmission for an automobile and the like and have already been in practical use. A continuously variable transmission including a torque convertor provided with a lockup clutch in a starting mechanism has also been on the market. Moreover, recently, a continuously variable transmission including a mechanism (a slip control) to intentionally slip a lockup clutch has been increasingly used in order to improve fuel efficiency in a lockup speed range and to attenuate shock in engagement of the lockup clutch. On the contrary, since self-induced vibration called shudder is likely to occur depending on a lubricating oil when such a slip control is conducted, an oil for a continuously variable transmission is required to have a sustainable anti-shudder performance.
However, since a high friction coefficient between metals is required in the continuously variable transmission, it is difficult to sustain the anti-shudder performance that is in a trade-off relationship with the friction coefficient. Particularly, since a slip time of the lockup clutch in a recent continuously variable transmission is set longer than that of a conventional lockup clutch, an anti-shudder lifetime needs to be further prolonged.
Accordingly, there has been proposed a lubricating oil composition containing a base oil, (a) alkaline earth metal sulfonate or alkaline earth metal phenate, (b) an imide compound and (c) a phosphorus compound so as to increase a friction coefficient between metals and simultaneously prolong the anti-shudder lifetime (see Patent Literature 1). Moreover, for the same reason, there has been a proposed a lubricating oil composition containing: either phosphate or phosphite: and tertiary amine (see Patent Literature 2). Further, there has been proposed a lubricating oil composition containing overbased Ca sulfonate, phosphite, primary amine and tertiary amine (see Patent Literature 3).